Priority is claimed on Japanese Patent Application No. 2004-351632, filed Dec. 3, 2004, the content of which is incorporated herein by reference.
1. Field of the Invention
This invention relates to an optical component manufactured using pulsed laser beam, and in particular pulsed laser beam the time duration of which is 10−12 second or less, such as femtosecond (10−15 second) pulses, and in particular relates to an optical low-pass filter suitable for solid-state image capture elements such as are used in video cameras, digital still cameras, fiberscopes and similar, as well as a manufacturing method for same. Such components are also suitable for use as optical low-pass filters in liquid crystal, plasma, EL, SED and other displays, and other dot-matrix display devices.
2. Description of Related Art
The spread of digital video cameras and digital still cameras in recent years has been accompanied by progress toward greater compactness of optical systems and higher pixel densities.
The digital video cameras and digital still cameras have solid-state image capture elements with pixels arranged in a discontinuous but regular array. Through spatial optical sampling of the object image, an image capture output is obtained corresponding to each of the pixels of the object image.
In a solid-state image capture element which thus optically samples the object image, the fineness of patterns which can be handled is determined in relation to the sampling frequency; if spatial frequency components at frequencies higher than the Nyquist frequency (hereafter called the “cutoff frequency”), which is one-half the sampling frequency, are included in the object image, then spurious signals due to aliasing artifacts may occur, so that, for example, in a color video camera, colors unrelated to the colors of the object may appear as such spurious signals in the reproduced images. Hence in devices having such solid-state image capture elements, optical low-pass filters which limit the object high spatial frequency components are positioned in the image capture optical system, to prevent the occurrence of spurious signals due to aliasing artifacts.
As such optical low-pass filters, conventional filters are widely known which utilize the birefringence of quartz to maintain contrast as high as possible with respect to lower spatial frequency components than the cutoff frequency.
Also in the prior art, focusing on the fact that the autocorrelation function of the pupil function of a focusing optical system provides the transfer function for the system (the absolute value of which is hereafter abbreviated “MTF”), a phase-type optical low-pass filter has been proposed in which the pupil function is intentionally provided with aberration to obtain the target optical characteristics. That is, the MTF represents the contrast with the spatial frequency, and so by inserting into the optical system an optical component which provides an MTF characteristic such that the contrast in the high spatial frequency region, above the cutoff frequency determined by the pixel aperture width and pitch, is made low, spurious-signal images due to aliasing are made less prominent.
As such a phase-type optical low-pass filter has, for example, a component which is a transparent substrate of glass, resin or similar, with a stripe-shape periodic structure formed on the surface thereof and is inserted into an optical system. Differences between the optical distances of transmitted light, that is, phase differences, are imparted by the stripe-shape periodic structure, changing the phase terms of the pupil function, to realize optical low-pass filter characteristics.
Various methods have been disclosed relating to the method of fabrication of such a phase-type optical low-pass filter. For example, a lithography method which uses semiconductor fabrication techniques is known. Here, a photosensitive resin film applied to the glass or other substrate is subjected to mask exposure using a photomask or to interference exposure, followed by development, to fabricate a photosensitive film pattern on the glass. Then, by performing dry etching treatment from above, a pattern is simultaneously etched in the glass exposed at the surface and in the photosensitive film; when at last the photosensitive film is consumed, depressions have been formed by etching in the glass surface, and the overall result is formation of a diffraction grating consisting of the glass substrate.
In addition, a method in which a pattern is created in metal or a single crystal by the above means, and this is used as a die in hot pressing and injection molding to transfer a pattern, as well as a method in which a photosetting resin is poured into such a die, and the resin is hardened into the shape of the die through irradiation with ultraviolet rays or similar, are also known (see Japanese Unexamined Patent Application, First Publication No. 6-308430).
In Japanese Unexamined Patent Application, First Publication Nos. 6-242404 and 7-5395, a method is disclosed in which a diamond wheel is used in grinding to cut a stripe structure with regular protrusions and depressions on a substrate surface.
In Japanese Unexamined Patent Application, First Publication No. 61-149923, a method is disclosed in which an ion-exchange method or similar is employed to form portions on a glass surface with different refractive indexes, to fabricate a phase-type diffraction grating.
On the other hand, advances in laser pulse compression technology in recent years have been accompanied by numerous reports on the fabrication of transparent materials using ultra-short-pulse laser light. In particular, the high peak power of laser light with pulse duration of femtosecond order are known to have made possible three-dimensional fabrication of the interior of transparent materials, utilizing multi-photon absorption processes. In Japanese Unexamined Patent Application, First Publication No. 9-311237, a method is disclosed in which laser light irradiation is used to form a high-refractive index region within glass, to form a three-dimensional optical waveguide. Further, Japanese Unexamined Patent Application, First Publication No. 2000-56112 discloses a method of using laser light irradiation to induce a permanent three-dimensional distribution of the refractive index within glass material, to create an optical diffraction element.
With respect to irradiation methods for femtosecond laser light, a fabrication method is known in which the pulse beam is focused by a lens and the focal point is scanned (in addition to the above, see Published Japanese Translation No. 2003-506731 of PCT International Publication), and in Japanese Unexamined Patent Application, First Publication No. 2004-196585, a method is disclosed in which a refractive index-modified area, with a two-dimensional or three-dimensional shape, is formed at once within glass or some other transparent material, using a laser beam without a scanning mechanism.
An optical low-pass filter which employs the birefringence of quartz incurs the expensive raw material costs of the quartz, and in particular requires a plurality of sheets of quartz and becomes thick when contemplating use in a color video camera using a solid-state image capture element, so that there are limits placed on the compactness of the optical system. Moreover, problems of precise optical axis alignment, and of strain when bonding sheets together, as well as numerous other problems related to manufacture make such components unsuitable for mass production.
On the other hand, conventional optical low-pass filters employing phase-type diffraction gratings, obtained by fabrication the surface of a material, have the following problems from the standpoint of device manufacture.
For example, when using lithography, there are numerous processes as explained above, and the time required leads to increased fabrication costs. On the other hand, when an attempt is made to for example fabricate a rectangular-shape grating with large step height differences, in consideration of the ease of fabrication, it is not easy to machine grooves which are deep in the vertical direction. Further, it may be necessary to select optimal environment conditions for dry etching depending on the material, and in other respects also control is complicating, and there is little degree of freedom in choice of materials. In addition, due to the nature of the fabrication method, fabrication of the surface is limited to two dimensions, so that there is little degree of freedom in the design of the structure.
On the other hand, when using a mold fabricated using lithography, mass production is improved compared with cases in which lithography is used to directly machine a substrate, and so there is the advantage that costs can be kept low. However, limitations are imposed on the selection of materials. That is, in the case of a hot-pressed replica, the material for fabrication is limited to glass and resin. In the case of the photopolymer method described in Japanese Unexamined Patent Application, First Publication No. 6-308430 also, the material is limited to a photosensitive resin. Further, in the case of a hot-pressed replica, when choosing the mold and material for fabrication, the durability of the mold with respect to the softening temperature of the glass is an issue; and conversely, the material should be preferably selected with this born in mind.
When cutting using a diamond wheel, mass production properties are poor compared with molding of glass or resin using a mold, and there are problems with fabrication multiple-angled or curving filter patterns. Accordingly, there is little degree of freedom in choosing shapes at the design stage, and shape precision is poor. Moreover, the mechanical strength of the material becomes an issue, so that there is little degree of freedom in choice of material as well.
Methods in which ion exchange techniques or similar are used to form portions on a glass surface the refractive index of which differs are in essence material surface fabrication methods, and so there is little degree of freedom of pattern design. In the case of ion exchange methods, a metal mask or similar must be formed by the above-described photolithography in order to obtain the desired pattern. This is performed by immersion in a fused-salt reactor, and the processes involved are complex.
On the other hand, with respect to internal fabrication using the above ultra-short pulse laser light, the above-mentioned Japanese Unexamined Patent Application, First Publication No. 2004-196585 describes a number of examples of optical components for optical communication, fabricated by forming regions in which the refractive index changes according to three-dimensional shapes. Further, the above-mentioned Japanese Unexamined Patent Application, First Publication No. 2000-56112 describes a method of fabrication of a three-dimensional volume-type diffraction grating, with applications as a Bragg diffraction grating employing regions in which the refractive index changes in layers. However, in none of these disclosures are applications to optical components which control the phase of light transmitted therethrough, and in particular to optical low-pass filters, discussed.